This invention relates to an improved system and method for performing switching in an implantable medical device; and more specifically, relates to the use of Micro-Electrical-Mechanical systems (MEMs) technology to implement switching circuitry of an implantable medical device.
Many Implantable Medical Devices (IMDs) include circuits for delivering electrical stimulation to tissue. For example, implantable pacing, defibrillation, and cardioversion devices are designed to deliver electrical stimulation to the heart via electrodes that are in contact with cardiac tissue. Other types of implantable devices such as neuro-stimulation systems are known for delivering electrical stimulation to muscle, nerve, or other types of tissue within a patient""s body.
IMDs that deliver electrical stimulation generally include output switching networks to selectively couple stimulation energy to cardiac, muscular, or neurologic tissue from batteries and/or capacitors under supervisory control of algorithms or firmware resident in the device. In the prior art, these switches are generally implemented in CMOS technology using CMOS Field Effect Transistors (FETs). These transistors can be readily implemented in silicon devices using three to five-micron, or larger, CMOS technology. However, as the feature size of the CMOS FETs is decreased below three microns, the breakdown voltage of the FETs is also decreased. If the breakdown voltage decreases to a voltage that is at, or near, the voltage that will be applied across a FET, stimulation pulse parasitic leakage will occur, causing ineffective stimulation, increasing battery current drain, and potentially resulting in damage to the integrated circuit.
One proposed mechanism for solving the above-described problem involves implementing all switching circuitry in at least a three-micron technology in a first integrated circuit, while implementing all other circuitry for the IMD in another integrated circuit employing smaller-sized gates. This type of approach is described in U.S. Pat. No. 5,833,710 to Jacobson. This proposed solution adds an additional integrated circuit to the design, increasing system size and cost. Moreover, this method requires the addition of hybrid circuit interconnects to couple the multiple integrated circuits. These interconnections are costly to manufacture and are prone to failure. Also, interconnections on the hybrid circuit level generally consume more current than interconnections contained within a single integrated circuit.
Another solution to the problem involves employing several FET transistors in series in place of a single FET to implement a switching function. This allows a given voltage drop to be shared by multiple transistors such that the likelihood of circuit damage and/or leakage is decreased. However, this solution has the disadvantage of greatly increasing the amount of silicon area required to implement each switch. Additionally, the design is complicated because the multiple FETs implementing a single switch must be enabled in a predetermined order to prevent the full voltage drop from being experienced by a single FET even for a very brief period, since this could damage the circuit or cause large leakage currents. The implementation of this design approach therefore generally results in the use of a significantly increased silicon die area.
Yet another approach is discussed in U.S. Pat. No. 5,097,830 to Eikefjord, et al. This patent describes an external defibrillator that incorporates transfer relays to deliver the defibrillation pulse to a patient. This design consumes a relatively large amount of space.
While the above discussion focuses on switching networks used within output circuitry of an IMD, those skilled in the art will recognize that other switches in an IMD are associated with problems similar to those discussed above. What is needed, therefore, is an improved switching system and method for use in implementing any switching function within an IMD that can be robustly implemented using a substantially smaller die area.
The current invention involves an improved switching system for use with an implantable medical device (IMD). The system utilizes Micro-Electrical-Mechanical system (MEMs) switches in place of one or more switches conventionally implemented using transistor networks. These MEMs switches provide electrical and mechanical coupling between two terminals of a circuit. These switches, which have dimensions in a range of less than 10 microns, can be manufactured on conventional integrated circuit dies. Because these MEMs switches are capable of sustaining a much larger voltage across the switch terminals than are conventional switches implemented using transistor networks, the resulting circuit is more reliable, and the switching circuit and control logic may be simplified. This minimizes the die area required to implement the system. If desired, an entire IMD including switching circuitry may be implemented using a single integrated circuit die.
According to one aspect of the system, the fabrication of the MEMs switches may be performed using one or more separate tubs or wells on a silicon substrate. This isolates switching circuitry from other IMD circuitry. As such, switching circuitry implemented using three to five micron technology may reside on the same substrate as transistors that are implemented using smaller technology. Isolating the circuits in this manner minimizes substrate crosstalk, breakdown, heating, and circuit latch-up concerns. This approach could also be used to isolate RF or noise-sensitive circuitry.
Various types of switches may be implemented using MEMs technology, including latching and momentary-contact switches. The switches may be activated using various types of activation mechanisms including electrical, electromagnetic, and thermal signals. These switches may be fabricated using any of the known fabrication techniques, including the Lithographie, Galvanoformung, Abformung (LIGA) method.
According to one embodiment, the invention involves an IMD that is capable of providing electrical stimulation to a patient where the output switches are implemented using MEMs switch technology. In another embodiment, the invention involves an IMD including a first circuit that is capable of providing electrical stimulation to a patient, and a switching circuit including a MEMs switch that selectively allows the electrical stimulation to be routed to the desired electrode pair or configuration on the patient. The first circuit may be a circuit to deliver pacing pulses, may be a high-voltage output circuit as may be included in a defibrillation system, may involve a neurostimulator, or another type of treatment mechanism.
In a further embodiment the output circuit implemented in the IMD may include a return current path that is selectable using switches implemented using MEMs technology. In an additional embodiment, the IMD may include a surge protection circuit implemented using MEMs technology, where a switch or switches may open upon sensing a condition that may damage the implanted device. In yet another embodiment, the invention may include a MEMs switch or switches used to selectively apply power to one or more circuits in an IMD.
According to one aspect of the invention, a method of controlling delivery of electrical stimulation to a body is provided, including the steps of generating a stimulation signal, and utilizing a MEMs switch to control delivery of that stimulation signal to the body. The MEMs switch may be controlled using any of the mechanisms described above, including electrical, electromagnetic, and thermal control systems.
Additional objects, features, and advantages of the present invention will become apparent from the description and the related drawings, and from the claims.